Integrated circuits are formed on semiconductor wafer substrates by a number of processing steps. These steps include deposition, etching, implantation, doping, and other semiconductor processing steps well known to those skilled in the art. Thin films are typically formed on the surface of the semiconductor wafer substrates (hereinafter referred to as wafer substrates) by a deposition process. The thickness of such films may range from about a few hundred angstroms to several micrometers. Often, three or more film layers are formed on the surface of a single wafer substrate.
In fabricating wafer substrates, it may be desirable to minimize or control stresses in surface films. High surface stresses can cause, for example, silicide lifting, the formation of voids or crack and other conditions that adversely affect semiconductor devices (i.e., chips) which are fabricated on the wafer substrates. In practice, surface stresses become more problematical as the level of circuit integration increases and are especially troublesome when fabricating large scale integration (LSI), very large scale integration (VLSI), and ultra large scale integration (VLSI) semiconductor devices.
The stress in the surface film of the wafer substrate can be either compressive or tensile. Assuming the film is on top of the wafer substrate, a compressive stress in a surface film may cause the wafer substrate to slightly bow in a concave direction, while a tensile stress in a surface film may cause the wafer substrate to slightly bow in a convex direction. Therefore, both compressive and tensile stresses cause the surface of the wafer substrate to deviate from exact planarity. The extent of the deviation from planarity can be expressed in terms of the radius of curvature of a surface of the wafer substrate. In general, the greater the magnitude of surface stresses, the smaller the radius of curvature.
Due to the aforementioned problems caused by stresses in surface films on wafer substrates, it may be desirable to measure such stresses. The measurements can be used, for example, to identify wafer substrates that are likely to provide low yields of semiconductor devices or which might produce devices prone to early failure. Generally, stresses in surface films are not measured directly but, instead, are inferred from measurements of the radius of curvature of the surface of interest. Various techniques are available that enable measuring of curvature and consequently stress.